Review of Answer Choices (Correct Answer in Green):
● Acral lentiginous melanoma
● Verruca vulgaris (plantar wart)
● Pigmented basal cell carcinoma
● Subcorneal hematoma
● Tinea nigra
Explanation:
In this elderly patient of South Asian descent presenting with a new multicolored pigmented lesion, the working diagnosis should be acral lentiginous melanoma (ALM) until proven otherwise which is the most common subtype in skin of color. Although ALM is the least common subtype of melanoma overall (about 5% of all melanomas), it is the most common subtype found in patients with darker complexions, such as those from African (up to 70% of those diagnosed with melanomas), Asian (up to 40%), and Hispanic ancestries. Studies have found that UV exposure does not play a significant role in its development, and it does not present with the same risk factors as the more common melanomas. As such, it can often be overlooked and misdiagnosed in populations with darker skin tones. It is most commonly found on the lower extremities and is often advanced at the time of presentation, leading to a high level of morbidity and mortality. Moreover, this patient has lupus and is on long-term immunosuppressant therapy, increasing his risk of malignancy.
In this case, it is also important to keep other pigmented variants of common skin malignancy, like basal cell carcinoma (BCC), high up on the differential. Another important differential to consider is tinea nigra which is caused by Hortaea werneckii. It is usually asymptomatic, well-demarcated lesion of variable size and irregular shape (between 1-5 cm). There is often a single brown or black macule on the palmoplantar skin. Many times, the lesion is misdiagnosed as malignant melanoma.
Common warts present as papules with a rough, papillomatous and hyperkeratotic surface ranging in size from 1 mm to larger than 1 cm. A tiny black dot may be observed in the middle of each scaly spot, due to an intracorneal haemorrhage. Subcorneal hematoma is blood accumulation underneath the horny layer of skin. It usually occurs in the palmoplantar region as a consequence of blunt trauma. The history and morphology does not fit here for plantar wart or subcorneal hematoma.